Hyperthermia may reduce or eliminate cancerous tumors. Stable, temperature controlled heating with Nd:YAG laser light, through a fiberoptic probe inserted in cancerous tissue has been demonstrated for periods of more than 30 minutes.The objective of this research is to develop a multiple fiberoptic system with feedback controlled temperature in the tumor. The entire volume of the cancerous tissue could then be uniformly heated to a specified temperature. Nd:YAG lasers are available at many medical centers. The output from these lasers can be anywhere from 10 to 100 watts and the power to heat the tumor is approximately 2 watts. During Phase I the investigator's will design and test a four fiber system which will distribute the output from a single Nd:YAG laser into the cancerous tissue and regulate the temperature of the tissue to the prescribed limits. Temperature profiles will be analyzed and heating requirements for hyperthermia treatment will be determined. The results of these tests will be incorporated into a design for a commercial prototype to be fabricated in Phase II. Extensive clinical tests would be carried out in Phase II and success would lead to a commercial Nd:YAG, accessory for hyperthermia treatment of cancerous tumors.